1. Field of the Invention
The present invention relates to a pressure-sensitive adhesive optical film. The present invention also relates to an image display, such as a liquid crystal display, an organic electroluminescent display, a cathode ray tube (CRT), and a plasma display panel (PDP), using the pressure-sensitive adhesive optical film.
The pressure-sensitive adhesive optical film of the present invention has an discotic liquid crystal layer and is useful as an optical compensation film to improve display contrast and the viewing angle characteristics of displayed colors. In particular, the pressure-sensitive adhesive optical film having a polarizer laminated therein is useful as an elliptically polarizing plate with the function of optical compensation.
2. Description of the Related Art
The liquid crystal display market has rapidly expanded in such fields as clocks, mobile phones, PDAs, note PCs, PC monitors, DVD players, and TVs. Liquid crystal displays use liquid crystal switching to visualize changes in polarization state, and based on the display principle, they use polarizers. Particularly in TV applications and the like, there is an increasing demand for high brightness and high contrast display, and thus brighter (higher transmittance) and higher contrast (higher degree of polarization) polarizers have been developed and introduced.
The currently dominating type of general liquid crystal displays is a TFT-LCD using a TN liquid crystal. This type has the advantages of a high response speed and a capability of producing high contrast. However, if displays on TN liquid crystal panels are viewed at angles oblique to the normal direction, their contrast can be significant low, or gradation reversal, an event in which the gradation of displays is inverted, or the like can take place. Thus, TN liquid crystals have very narrow viewing angle characteristics. On the other hand, applications such as large PC monitors and large screen televisions should satisfy certain requirements such as high contrast, wide viewing angle, and little fluctuation in display colors over viewing angles. Thus, TN mode TFT-LCDs for use in such applications must have a retardation film for compensating for viewing angles.
Conventionally, stretched birefringent polymer films have been used as the retardation film. Recently, it has been proposed that the optical compensation film made of such a stretched birefringent film is replaced with another optical compensation film having an optically anisotropic layer formed of a liquid crystalline molecule on a transparent support. Since liquid crystalline molecules can have various orientation modes, the use of liquid crystalline molecules has enabled the achievement of certain optical properties that had not been achieved with conventional stretched birefringent polymer films.
For example, one of the proposed retardation films for viewing angle compensation is Wide View Film manufactured by Fuji Photo Film Co., Ltd., which uses a discotic liquid crystal with negative refractive index anisotropy (see JP-A No. 08-95032 and JP-B No. 2767382). This retardation film includes a transparent base film and a discotic liquid crystal layer that is provided on one side of the base film and has an obliquely-oriented optical axis. The main purpose of this retardation film is to improve viewing angle characteristics in a state where a certain voltage for black viewing is applied. Specifically, in a state where a certain voltage is applied, a liquid crystal molecule in a liquid crystal cell shows positive refractive index anisotropy with an optical axis tilted with respect to a glass substrate. In order to compensate for the retardation caused by this refractive index anisotropy, the retardation film uses a liquid crystalline molecule having an optical axis tilted with respect to the film normal direction and having negative refractive index anisotropy.
In the retardation film for viewing angle compensation, a polarizer is laminated on the transparent base film to form an elliptically polarizing plate, and a pressure-sensitive adhesive is laminated on the discotic liquid crystal layer. The pressure-sensitive adhesive optical film having the pressure-sensitive adhesive layer laminated therein and serving as a retardation film, an elliptically polarizing plate, or the like is bonded to a liquid crystal cell or the like through the pressure-sensitive adhesive layer. The above-mentioned retardation film for viewing angle compensation or the above-mentioned elliptically polarizing plate may be bonded to a liquid crystal cell or the like through the pressure-sensitive adhesive layer to form a liquid crystal display.
Acrylic pressure-sensitive adhesives including an acrylic polymer as a base polymer are frequently used for pressure-sensitive adhesive-attached optical films, because of their good adhesion, transparency and so on. Methods for crosslinking acrylic pressure-sensitive adhesives use isocyanate crosslinking agents and a combination with the copolymerized functional monomers to the acrylic polymer in many cases.
Such optical films are bonded to liquid crystal cells to form liquid crystal panels, which are used and incorporated into liquid crystal displays. Liquid crystal displays have been used at first for calculators and later for televisions, monitors and so on. Liquid crystal displays are placed under various conditions such as hot and/or humid conditions and thus required to have high durability such that display quality degradation can be prevented.
When liquid crystal displays are placed under hot and/or humid conditions, however, display unevenness can sometimes occur in the peripheral portion of the liquid crystal panel to cause display defects. The display unevenness in the peripheral portion can be significantly found, particularly when the above-mentioned retardation film for viewing angle compensation or the above-mentioned elliptically polarizing plate is used.
In order to improve the display unevenness in the peripheral portion, it is proposed that a pressure-sensitive adhesive composition containing a plasticizer or an oligomer component should be used for the pressure-sensitive adhesive-attached optical film (see JP-A No. 09-87593 or JP-A No. 10-279907). However, such a pressure-sensitive adhesive composition has a problem in which the dopant such as the plasticizer or the oligomer component can be precipitated to cause defects in appearance or degradation of the pressure-sensitive adhesive in a long-time heating test.